Acoustic position finder



5 Sheets-Sheet 1 Apnl 5, 1960 M. KEISER ACOUSTIC POSITION FINDEROriginal Filed April 23, 19 53 3\ mobdmzuo 2.) 2.3 5.2523

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MORR5 KHSER HUEDOW ATTORNEY P 1960 M. KEISER ACOUSTIC POSITION FINDEROriginal Filed April 23. 1953 FIG. 2.

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ELECTRONIC ATTORNEY April 5, 1960 M. KEISER ACOUSTIC POSITION FINDEROriginal Filed April 23, 1953 5 Sheets-Sheet 4 l OMB INVENTOR.

MORRIS KEISER 3/111. ATTORNEY April 5, 1960 M. KEISER ACOUSTIC POSITIONFINDER Original Filed April as. 1953 5 Sheets-Sheet 5 INVENTOR.

MORRIS KEISER BY W4 ATTORNEY United States Patent O F ACOUSTIC POSITIONFINDER Morris Keis'er, Cocoa Beach, Fla., assignorto the United Statesof America as represented by the Secretary of War Continuation ofapplication Serial No. 350,784, April 23, 1953. This application July22, 1957, Serial No- 673,546

9 cram. (Cl. 340-46) (Granted under Title 35, Us. Code 1952 see. 266)This application is a continuation of application Serial No. 591,763,filed May 3, 1945 and its continuation application Serial No.- 350,784,filed April 23, 1953, both now abandoned.

This invention may be manufactured and used by or for the Government forgovernmental purposes, without the payment of any royalty thereon.

This invention relates to an acoustic position-finder having a pluralityof soundpickup stations, the'stations being separated or displaced withrespect to each other. Acoustic position-finders of this type may beused for 10- cating the positions of such sources of sound as set offexplosions, enemy guns, 'etc., each station giving the azimuth of thepicked up sound wave, and the position of the source of sound beingdetermined by obtaining the intersection of the azimuth lines;

According to one embodiment of theinvention, the position-finderincludes at least two sound-intercepting stations disposed in spacedrelationship at each end of a base line, each station having atriangular array with a microphone placed at each apex of the triangle.The sound wave intercepted by the three microphones of the array istransformed into three corresponding voltage signals which areamplified, and the outputs of these amplifiers are impressed on threeindividual, magnetic recording heads positioned in recordingrelationship with respect to three magnetic tapes, the tapes acting as arecording medium for the intercepted sound waves; the number of magnetictapes thus corresponds to the number of microphones used in each arrayfor intercepting the sound wave. All tapes are mounted rigidly on theouter peripheries of cylindrical drums, which in turn are mounted on acommon shaft, and are driven at a substantially constant speed by meansof an appropriate governor-controlled or synchronous motor. All magneticrecording units are provided'with erasing magnetic heads, with theresult that the units record and erase continuously. When the sound thatis to be recorded is heard by an operator, the recording and erasingcircuits are disconnected by the operator, with the result that magneticrecords of the sound wave are retained on all magnetic tapes, theposition of the recorded signals on the individual magnetic tapes beingdetermined by the time of arrival of the sound waves at the arrays, andalso at the individual microphones of each array. The magnetic tapes orloops are of sufiicient diameter to provide the time reservoir requiredfor disconnecting the recording and erasing beads by the operators uponhearing the sound and prior to the erasing of the recorded sound wave bythe erasing heads. Three individual reproducing heads are provided forplaying'back the signals recorded on each recording channelrespectively. The trapped'sound records are used for producing thevisual images of the waves, the three images obtained from each arraybeing used for measuring the degree of linear displacement of the tworecords with respect to the third record of the same array. .Thisisaccomplished by align- 2,932,002 Patented Apr. 5, 1960.

ing the two reproducing heads with respect to their tapes until allthree heads reproduce the sound wave in identical phase relationship onsome suitable visual indicator of the signal, such as a cathode-rayoscilloscope. Suitable mechanical means, such as mechanical counters,are provided for measuring the displacements imparted to the twoadjustable reproducing heads, and the azimuth of the intercepted soundwave is determined from the reading given by the counters.

According to another embodiment of the invention, the visual indicatoris replaced with a headset, one receiver being connected to onemagneticreproducer, and the other receiver-to the other magnetic reproducer. Theoperator adjusts the position of one reproducer until the two soundsheard by him occur simultaneously.

According to an additional embodiment of the invention, the magnetictapes are stationary during the reproducing cycle, and the reproducingheads are rotated with respect to the stationary magnetic tapes. Thedegree of signal displacements on the tapes is measured by shifting thetape rather than the reproducing heads. Six individual magnetic tapesare used in one system, and in the other, the total number of magnetictapes has been reduced to two.

According to still another embodiment of the invention, the magnetctapes are not used, and the sought resuits are obtained wholly byelectronic means, each microphone array being connected to anoscilloscope which produces three traces of the intercepted sound wave;one trace for each microphone, and the azimuth of the sound wave isdetermined by measuring on the oscilloscope screen the displacements. ofthe two traces with respect to the third trace.

Field tests have indicated that the triangle method of sound-ranging issuperior to the other methods used in the past, the triangle methodbeing adapted to satisfy the more rigorous demands of present fieldoperations. These demands may be grouped as follows:

(a) Determination of maximum possible ranges with the lightestequipment, which at once calls for higher precision on the apparatusused for determining the azimuth of the sound wave;

(b) Greater speed in obtaining the results;

(0) Minimum noise-free time requirements for obtaining the results.

The first of these demands requires an improvement of the apparatus andgeneral instrumental technique which includes the method of reading therecord. The second and third demands require a more suitable microphonearray for speeding up the field installation, and for speeding up andsimplification of the entire operating technique. The apparatusdisclosed in the specification satisfies all of the above mentioneddemands more adequately than any of the prior known sound-ranging orposition-finding systems.

Determination of maximum possible range with the lightest equipmentattainable necessitates the reduction of the number of pickup devices,if such is possible, and the reduction of the size of the sound array.The invention discloses a sound array in which the number of the soundpickup devices has been reduced to three, the minimum required fordefining a plane, these devices being preferably positioned inthehorizontal plane, and at the apexes of an equilateral triangle, suchtriangle subtending the maximum area for the given total length of thesides of the triangle. Moreover, as will be demonstrated by themathematical equations in this specification, such array simplifies thedetermination of azimuth of the intercepted sound wave, since thedetermination does not require an absolute calibration of the timingsystem in the central station equipment, or the knowledge of the lengthof the sides of the triangle, so long as all sides are equal. Thepositioning of the sound pickup devices in the horizontal plane isdesirable to avoid the otherwise necessary corrections in thecomputations of the azimuth. The reduced size of the sound array isbeneficial for several reasons: since the length of the electricalcables is obviously proportional to the size of the array, any reductionin its size at once reduces the bulk of the cable, and thus the size ofthe entire equipment. Moreover, it is much simpler to install andoperate such array, which is an important factor, when such array mustbe operated in the vicinity of the front lines. From a strictlyfunctional point of view, however, the reduction in size of the array isextremely beneficial for obtaining identical wave-forms at all points ofthe array. I

There is an additional factor which must be considered when arrays ofthis type are operated in thevicinity of the front lines. The air overbattle fronts is filled with sounds, and, if the microphone array isgiven large dimensions, a large number of individual sounds coming fromdifferent sources may be recorded by the system in the interim of timewhich is necessary for the desired sound wave to travel across theentire sound array. It is obvious that the smaller the sound array, theshorter is the period of time necessary for any individual sound wave totraverse the entire array, i.e., the shorter is the operating time ofthe array, and, as a. consequence, the probability of interferencebetween the desired and undesired sound waves is at a minimum.

The reduction in size of the microphone array, however, imposes morestringent requirements on the measurement of the operating time of thearray, since this interval of time becomes increasingly shorter as thesize of the array is reduced. Accordingly, if the system is to have thedesired degree of accuracy in spite of the reduced size of the array,the apparatus for measuring this operating time must have sufiicientlyhigh accuracy to maintain the over-all accuracy constant.

The invention discloses an apparatus in which the sought time intervalis not measured in absolute units of time, but is measured indirectly bycomparing the time of arrival of the sound wave at the pickup device No.l, with the time of arrival of the same sound wave at the pickup deviceNo. 3, and the time of arrival of the same wave at the pickup device No.2 with that of No. 3, and deriving the azimuth from the ratio of themeasured quantities. Thus it is not necessary to have the absolutecalibration of the timing system which greatly relaxes the functionalrequirements imposed on the system. i

Because of the fact that the azimuth of the incoming sound wave isdetermined in the manner indicated above, only a short period ofconstant speed of the recording medium is required, i.e., the speed mustremain constant only for the periodof time which is'required for thesound wave to traverse the microphone array. Since the microphone arrayhas now been reduced to a relatively small size, this interval of timehas been also reduced to a very short interval of time, and in actualpractice it is in the order of at most fifty (50) milli-seconds. It isobvious that even when the speed does not remain strictly constant overa long period of time, it is a relatively simple matter to obtain auniform speed for a period of fifty (50) milliseconds, since the inertiaof the moving parts alone, aside from the speed regulation which is alsoused, will resist any appreciable speed changes in such short intervalsof time. Thus the invention discloses a system which enablesmeasurements of time intervals which are in the order of one tenthousandths of a second (.0001 sec.), with an extremely simple equipmentwhich does not call for any absolute standards in any part of theequipment.

It is, therefore, an object of this invention to provide a method andapparatus for accurately determining the azimuth of a sound waveintercepted at a pickup station, which is provided withthree-sound-intercepting devices,

said devices being positioned at the apexes of an equilateral trianglelying in the horizontal plane, and instrumentalities connected to saidarray for comparing the time of arrival of the sound wave at therespective soundintercepting devices, whereby determination of azimuthis performed independent of the absolute time or distance measurements,and without absolute speed control in the devices used for measuring thephase relationship, since the azimuth determination is a function onlyof the ratio of the time dilierences in the arrival of the sound wave attwo points as compared to the third point.

An additional object of this invention is to provide a method andapparatus for accurately determining the azimuth of an intercepted soundwave at a pickup station provided with at least three sound-interceptingdevices, positioned preferably in the horizontal plane, so that theintercepting device's form-a planar sound array.

It is an additional object of this invention to provide a method andapparatus for accurately determining the azimuth of an interceptedsound'wave at a pickup station provided with three sound-interceptingdevices positioned in a single plane, so that said intercepting devicesform a plane microphone array, the array forming an equilateral triangleof small area so that the noise-free time requirement of the array forobtaining results is in the order of .050 second.

Still another object of this invention is to provide a method andapparatus for accurately determining the azimuth of an intercepted soundwave by means of three sound-intercepting devices, forming anequilateral triangle and a planar sound array of the system, each ofsaid sound-intercepting devices being connected to a magnetic tape forrecording the intercepted sound on said tape, the phase relationshipbetween the three records produced onthe magnetic tape corresponding tothe time of arrival of sound at the respective microphones, whereby thedetermination of the azimuth of the sound wave is reduced to themeasurement of the linear displacement of the two records with respectto the third record.

It is an additional object of this invention to provide a method andapparatus for accurately determining the azimuth of an intercepted soundwave at a pickup station provided with three sound-intercepting devices,said devices forming a horizontal planar array of said station, andinstrumentalities connected to said array for comparing the time ofarrival of said sound wave on the respective microphones, the azimuth ofthe incoming sound wave being determined solely from the ratio of thetime difference between the arrival of the sound wave at the twosound-intercepting devices as compared to the time of arrival of thesame sound wave at the third sound-intercepting device.

It is an additional object of this invention to provide a method andapparatus for determining the position of a source of sound waves byderiving at each station the azimuth line of the intercepted sound wave,and determining the position of the source of sound by obtaining thepoint of intersection of the azimuth lines.

These and other features of the invention will be more clearlyunderstood from the following detailed description and the accompanyingdrawings, in which:

Figure 1 is a block diagram of one embodiment of the position-finder;

Figure 2 illustrates an equilateral triangle, microphone array andgeometry of the problem which it solves;

Figure 3 is a modified form of Fig. l, the modification residing in thesubstitution of the earphones for the oscilloscopes used in Fig. 1;

Figure 4 is a block diagram of a position-finder wherein the reproducingheads are rotated and the tapes are stationary during the reproducingcycle of the system;

Figures 5 and 6 are cross-sectional views of the idlers and scanningwheel used with the finder of Fig. 4; the

cross-sectional views takenalong linsIS-i and shown in Fig. 4; a v

Figure 7 is a modified form of Fig. 4, the modification residing inreducing the number of magnetic tapes to two, as compared to six tapesused in Fig. 4;

2 Figure 8 is a block diagram of a position-finder Wherein allsteps areperformed electronically by means of the electronic switches and twoOscilloscopes, and the magnetic tapes are not used;

Figure 9 is an enlarged view of one of the oscilloscope screens alsoillustrated in Fig.8, and

Figure 10 is a modified block diagram of Fig. 1, the modificationresiding in the use of the recorded signals, the amplifiers beingswitched for reproducing in such a manner as to eliminate the phaseshift error.

- Referring to Fig. 2, it illustrates the equilateral triangle array,the pickup microphones being positioned at the apexes l, 2, 3 of thetriangle. -The determination of the azimuth of sound approach is'basedon the following conditions: (a) That the dimensions of the array aresmall compared to-the range so that the wave front of the sound approachis a plane and a correction for curvature of the wave front isnegligible.

(b) That the speed of sound (Scalar) and the vertical angle of approachof the wave front is constant for the time of traverse of the sound overthe array (approximately 50 milliseconds). The trace velocity V (theapparent velocity of sound in the plane of the array) is thereforeconstant.

In the configuration shown there are three microphones A B C at thedistance ABEBC=AC, but to provide a more general solution the microphoneC is also referred to as C and distances are separately recited,Azimuths of sound source and microphones are designated relative solveone of these equations, such as 2, forthe angler V(T A T,,) (3) but thisrequired accurate determination of the time, distance, and horizontalcomponent of the velocity of sound, including meteorologicalobservations of the temperature, humidity, etc., and considerableobservation and experience in determining elevation angle of sounds fromthe sources of interest, and still gave only a very questionableaccuracy. The present technique involves only a ratio, such as divisionof Equation 1 by Equation 2 as follows:

Cos (OCA) .T, T, CB Substituting ratios R R to reduce length ofcalculations in place of Tb m T,-T,, CB V Cos OCB cos 0+sin OCB sin 0 RCos OCA cos 6 +sin OOA sin 6 1 2 Cos OCB-I-srn'OC'B tan 0 =R1R2 V I (-6)+R1R3 sin tan a 0 aretan 6= arctan (10 Another convenient calculationwould assume 0CA=O, with same array OCB would be 60 and:

Another convenient array, in the form of a right isosceles triangle orsquare as in Hubbard Patent No. 1,645,810, might have 0C'A=0, OCB=,C'A=CB, in which:

0=arctan =arctan =arctan R1 Arrays having unequal sides would requireuse of the ratio R of the full formula.

By observation it would be obvious that differences must not be measuredalong nearly parallel lines. The formula also shows that the resultswould be indeterminate since R R would be nearly unity, sines nearlyequal, and therefore actual value of the ratio would be nearly 0/0 andboth the ratio and the angle defined by the arctan of such ratio wouldbe indeterminate.

In the array as shown the greatest accuracy is obtained by using theratio of the largest to the intermediate time diiference. The azimuthline is drawn through the midpoint of that side of the triangle forwhich the time difference of arrival of sound between the ends is aminimum as shown in Fig. 2.

This angle is obtained directly by means of a slide-rule which has itsscales arranged to give a direct answer for 0 in degrees and minutes ofthe angle. Ordinary slide rules in which the ratio is computed include ascale suit able for this purpose designated the T or tangent scale.

Referring to Equation 4 it is seen that the determination of 0 isdependent only on a ratio of time difference. It therefore follows:

(a) that it is not necessary to have an absolute calculation of thetiming system, and it is only necessary that the system operates atuniform speed during the interval of time required for the sound wave totraverse the array.

(12) that it is not necessary to know the length of the sides of thetriangle-it is only necessary that they be of known ratio and that thearea encompassed by the array be large enough to give adequatesensitivity in the measurement of 0, but not so large as to make theassumptions made in (a) and (b), above, untenable.

(c) that the size of the array does not enter into the solution of theequation for azimuth, but that the configuration of the array isimportant. It is to be noted however, that the accuracy of the finalresults is a function of the absolute size of the array, since themeasurements of the relative displacements of the respective recordswith respect to each other on the magnetic tape, if such is used, or onthe screen of the oscilloscope, if such measurements are performedsolely by means of the 7 oscilloscope screen, are directly affected bythe absolute size of the array, shorter and shorter measurements beinginvolved as the size of the array is reduced.

Therefore, to sum up the basic principles of the system, it isespecially suitable for field use which requires portability andsimplicity of the equipment. The necessary reduction of the size of thearrays would impose great demands on the accuracy of the entire systemand the precision of the individual components if absolute measurementswere required, these demands are circumvented by the disclosed systemthrough the use of a small array positioned in a horizontal plane andhaving a form of an equilateral triangle which reduces the solution ofthe azimuth determination to the ratio of the time differences inEquation 4. Therefore, demands, on space and time measurements, and, inturn, requirements of the constancy of speed in the recording apparatusare greatly relaxed, and, as a consequence, it becomes possible toreduce equipment to a simple combination of instrumentalities which canbe very readily used in the field.

Referring now to Fig. 1, it illustrates two microphone arrays 10 and 12placed at the ends of a base line 14. The arrays are in the form ofequilateral triangles with the microphones through placed at each apexof the triangle. The triangular array has a fairly uniform sensitivityto all azimuth angles, so that the orientation of the array may besuited to the terrain, except that, the plane of the triangle should beas nearly horizontal as possible to avoid the necessity of applying thetilt correction to the obtained results when the array is not in ahorizontal plane. The base line 14 should be approximately parallel tothe plane of the incoming sound wave for maximum accuracy in determiningpositions of the sources of sound, the larger is the angle between thetwo azimuth lines, the higher is the accuracy of the results. The lengthof the sides of the triangle are formed by the microphones 15 through20, and the degree of separation between the array stations, i.e., thelength of base line 14, and the dimensions of the array, i.e., thedistance between the microphones in each array, are all controlled bythe range to be measured and the desired accuracy of theposition-finder. When the position-finder is used for determining thelocation of relatively remote objects, the base line, as well as thespacing between the microphones in each triangular array, should belonger than when the position-finder is used for locating short rangessuch as those encountered in connection with the determination of thelocations of such objects as mortars and machine guns, i.e., the size ofthe array and the base line should be commensurate with the ranges thatare to be measured. The following illustrative example is given toexemplify the order of the magnitudes of the above mentioned quantitieswhich give satisfactory results:

For 1000 yard range (small arms) Sides of triangle yds 5 Baseline yds..500

Since the recording and reproducing apparatus used in connection witheach array is identical, it will be sufficient to describe only one setof the recording-reproducing channels,and it will be done in connectionwith the left array 10. The microphones 15, 16 and 17 are connected toamplifiers 22, 24 and 23. The amplifiers are grounded through a commonconductor 21 and a switch 49, this switch being used for disconnectingthe recording amplifiers from the array immediately after the desiredsound wave has been recorded on the magnetic tapes. This commonconductor also extends to the amplifiers 25, 26 and 27 of the rightmicrophone array 12, so that all recording channels become disconnectedsimultaneously when switch 40 is opened.

The audio amplifiers 22, 23 and 2d are connected to the magneticrecording heads 28, 2 and 30 which are positioned in recordingrelationship with respect to the magnetic tapes 34, and 36. The rightarray 12 is provided with the similar amplifiers 25, 26 and 27, recording magnetic heads 31, 32 and 33, and magnetic loops 37, 38 and 39. Eachof the magnetic loops 34 through 3? consists of a thin, flat magnetictape, this type of magnetic tape having the advantage of permittingtwosided recording, i.e., the magnetic core of the recorder has a smallair gap, and the tape passes through this air gap. All magnetic loopsare mounted rigidly on a shaft which is connected through a gear box 42to a motor which rotates the loops at a substantially uniform angularvelocity. Motor 50, as mentioned previously, is either a D.C.,governor-controlled motor or a small synchronous motor, if a source ofA.C. is available. All magnetic loops are provided with erasing heads 51through 56 which are connected in parallel through a common conductor 57to the left side of switch 40 and supersonic oscillator 58. Thisoscillator impresses a signal of sufficient intensity on the erasingheads 51 through 56, to erase any recorded signals on the magneticloops. Since oscillator 58 operates at a supersonic frequency, it mayalso be used as a source of AC. biassing for the magnetic tapes duringthe recording cycle of the system, in which case, it is also connectedthrough an attenuator to the recording heads 28 through 33. Thisconnection is not shown in the figure. Since oscillator 58 is connectedto the erasing heads through switch 40, opening of the latter alsodisconnects all erasing heads, with the result that the desired soundwave record is retained on the magnetic loops.

All the magnetic tape loops are provided with repro' ducing heads 60through which are used for reproducing the recorded signals on thescreens of cathode-ray Oscilloscopes 66 and 68. These may be connectedto any of the reproducing heads of one array through a threepositionswitch 69 or 70 and suitable amplifiers 71 and 72. The sweep circuits ofthe oscilloscopes are controlled by means of commutators 74 and 76mounted on shaft 45. These commutators thru pickup brushes 81 and 83supply the necessary triggering pulses for the saw-tooth generators 78and 80, the outputs of which provide the time axes for theOscilloscopes. In order to adjust the time axis on the oscilloscopescreens, commutating brushes 82 and 84 are mounted on adjustablesegments 85 and S6 which may be rotated around shaft 45, and thus movethe position of the saw-tooth waves with respect to the reproducedsignals, which is equivalent to shifting the latter on the screen of theOscilloscopes.

When the operator of the position-finder hears the expected sound, suchas firing of a distant gun or a burst of machine gun fire, he opensswitch 40, and, since his time of reaction to the sound is in the orderof 0.3 to 0.5 second, sufficient time elapses between the actualrecording of the sound wave and disconnecting of the recording channels,with the result that the recording channels record the desired soundwave on the magnetic loops before they are disconnected. At the sametime, the dimensions, or the length of the recording loops is made toprovide the necessary time reservoir, so that the recorded sound remainson the tapes and is not erased by the erasing heads prior to the openingof switch 40. Accordingly, the length of the magnetic loops iscontrolled by the time required for opening switch 40 by the operatorand the speed of motor 50 during the recording period. Since it is notnecessary to record and to reproduce faithfully the entire frequencyspectrum of the sound, the minimum recording speed of the magnetic loopsmay be relatively low, which diminishes the required diameter of themagnetic loops. This minimum permissible recording speed is determinedby first determining the useful range of frequencies composing thesought sound wave, and adjusting the speed so that the maximum frequencyis recorded faithfully on the tape. Thus the useful range of frequenciesmust be recorded and reproduced faithfully since it obviously determinesthe waveform of the sought signal. For mortar blas't second.

Opening of switch 40 completes the recording cycle of the systemand thedesired signals have now been trapped or recorded on all the loops inthe time relationships corresponding to the times of arrival of thesound wave at the respective microphones. It now remains only todetermine the difference between the positions of the signals on therespective loops, and this is accomplished by using the oscilloscopes 66and 68 in the respective array channels. The procedure for measuring thedifferences between the times of arrival of the sound waves in therespective channels is accomplished as follows: magnetic loop 35 isconnected through the reproducing head 61, switch 69, and amplifier 71to the vertical deflection plates of oscilloscope 66, and any desiredportion of the recorded sound wave 79 is positioned under the hairlineprovided on the screen of the oscilloscope. Intersection of the zeroaxis is used here as the point of'reference for aligning all waves withthe referenceline, As mentioned previously, the alignment of wave 79 isaccomplished by adjusting the angular position of sector 85 with respectto commutator 74 which shifts the saw-tooth wave either to the left orto the right with respect to the signal. Once the sound wave 79 has beenproperly positioned on the screen of the oscilloscope, sector 85 remainsfixed, and switch 69 is connected either to loop 36 or 34 forreproducing on the screen of oscilloscope 66 the signals recorded onthese loops in identically the same manner. Since the position of thesaw-tooth Wave now remains fixed, proper centering of the signals on theoscilloscope screen is accomplished by adjusting the time of appearanceof the signal on the vertical deflection plates of the oscilloscope bymanually operating sector 87 or 88. These sectors act as supportingbrackets for the reproducing heads 60 and 62, respectively,'and areconnected by means of Worm gears 89 and 90 to mechanical counters 91 and92, which indicate the number of turns given to the worm gears 89 and 90for positioning the signals recorded on the magnetic tapes 34 and 36 inidentical phase relationship with the sound wave 79. When this isaccomplished, the readings of the counters 91 and 92 are used inEquation 4 for computing the position of the azimuth line 200 indicatedin Fig. 2.

It has been mentioned previously that, in order to de crease thediameter of the magnetic loops, it is desirable to have the recordingspeed as low as feasible with the sound wave being recorded. Speed ofthe motor is determined by the frequency of the signal, the size of thearray, and the velocity of sound; the speed must be sufficiently high togive an instrumental resolution equal to the angular accuracy required.If the reproducing speed is the same as the recording speed, therepetition rate of the wave may not be high enough for producingpersistent images on the oscilloscope screen. Moreover, it is desirableto reproduce the entire sound transient on the screen, which means that,with the saw-tooth wave period adjusted to give high repetition rate,the reproducing speed of the tape must be higher than the recordingspeed. For these reasons, motor 50 is provided with a gear box 49 whichis shifted into a high speed position when the tapes are used forreproducing the recorded signals.

In order to obtain the highest degree of accuracy possible in connectionwith a small array, amplifiers 22 through 27 should be adjusted toproduce the images on the screen as nearly identical as possible. Thiscan be accomplished by phasing the recording channels so that all thetraces or signals for any one array take-off in the same direction,i.e., the phasing of the input and output circuits in the amplifyingchannels should be identical. Moreover, since the azimuth angle isdetermined by measuring the degree of linear displacement between therespective records on the individual magnetic tapes,

it is important that the complete recording channels,-

which include microphones 15, 16, 17, amplifiers 22,

23, 24, and the magnetic recording heads 28, 29, and 30 introduceidentical phase shifts. This may be accomplished by impressing anartificially created sound on all microphones simultaneously, and bycomparing the results on the screen of oscilloscope 66. If thedifference, in the takeoff times between the respective channels isgreater than can be tolerated with the desired accuracy of theapparatus, the phase shifts of the channels must beadjusted by changingthe parameters of the circuits in amplifiers 22 through 27. The channelsof any one. array are adjusted to about the same over-all sensitivity,so that the records of all the traces will have approxi-. Because thecharacteristics of. all the recording and reproducing channels areidentical,.

mately equal amplitudes.

or can be adjusted to identity, the traces on the oscillo: scope screenare also identical, which permits a more accurate determination of thetime difference. 10, the phase error introduced by therecording channelsis eliminated altogether, as will be described later.

The maximum excursions of the traces should be limited to the boundariesof the oscilloscope screen, so

that the record may be read with peaks and valleys, on

the points where the velocity of the cathode-ray beam is maximum, aspoints of identity. For longer ranges, the frequency spectrum of therecorded sound pulses is generally lower, and the initial excursion ofthe trace fromits zero position is more gradual. As a result, theinstant of take-01f is uncertain and difficult to determine, and whenthe sound wave has a form as indicated above, it is preferable to alignthe records of the wave by using the peaks or valleys of the sound wavesas the points of identity in determining the difference between thetimes of arrival of the sound at the respective micro phones. This isillustrated on the screen of an oscilloscope 68 in Fig. 1, with the peakof the sound wave How by a phase shift, resulting from the phaseaddition of signal and noise pulses, will be at a minimum.

In the acoustic, position-finding system disclosed in; Fig. 1, therecorded signals are reproduced by means of the reproducing heads and asingle reproducing amplifier connected between the reproducing heads andthe oscil loscope; recording heads 28 through 33 and one of therecording amplifiers, such as amplifier 23, in the left array andamplifier 26 in the right array for recording as well as for reproducingthe signals by interposing switches making such operation of thecircuits possible, and by mounting two of the recording-reproducingheads in each array, such as 28 and 30, on the adjustable sectors 87 and88. The disadvantage of such arrangement resides in the fact that aftereach determination of the location of a source. of sound the two heads28 and .30 will be found in the displaced positions with respect to head29, and it will be necessary to move them back to their originalpositions before making the next recording, if the next record-. ing isto be made with the counters 91 and 92 in zero reading positions.

An additional contemplated modification of the system disclosed in Fig.1 relates to the commutators 74 a'nd' 76 which furnish the synchronizingpulses for the saw'-' tooth generators 78 and 80. As illustrated in thefigure;

when brushes 81 and 82 (left commutator) connect a. source of potential73 to saw-tooth generator 78, the

In Fig.1

When this is the;

The system may be simplified by using the' condenser-discharging tube ofsaw-tooth generator 78 (or 80') is made conductive, the saw-toothgenerating condenser is discharged, and there is a resumption of thecharging period of the condenser. Thus a single sawtooth wave isgenerated for each revolution of commutator 74. Since the desired signaloccupies only a small part of the magnetic loop, the parameters of thesawtooth generator should be preferably adjusted to complete thegeneration of the linear portion of the saw-tooth wave in that intervalof time which is actually occupied by the signal, and remain idle duringthe remaining portion of the revolution of the loop. This mode ofoperation of the saw-tooth generator eliminates all extraneous signalsfrom the screens of the oscilloscopes, and produces a much faster sweepand the concomitant finer definition of the wave form of the desiredsignal. To accomplish this result, source 73 may be connected in serieswith the charging circuit of the saw-tooth generating condenser, and thewidth of segment 75 adjusted to produce the linear portion of thesaw-tooth wave of the desired duration. The condenser is then dischargedin a wellknown manner through a gas-filled tube which becomes ionizedwhen the voltage across the condenser reaches a predetermined value.

Fig. 3 discloses an additional modification of the position-findingsystem disclosed in Fig. 1. Only the reproducing portion of one array isillustrated in Fig. 3, and the connections between the array and Fig. 3are indicated by a dotted matching line 33 in Figs. 1 and 3.

The modification resides in substituting earphones 390 t foroscilloscope 66 (and 68 in the right array), and impressing the signalfrom tape 35 on the left earphone 30.2, and the signal from tape 36 onthe right earphone 384. As sector 88 is turned, the operator hears thesound produced by the earphones occur simultaneously in both ears. Thesame procedure is followed with the signal recorded on tape 34. Theobtained readings of the counters 91 and 92 are used in the same manneras those obtained in Fig. 1, i.e., they are used in Equation 4.Amplifiers 395 and 306 must have substantially the same frequencyresponse characteristics, gain, and phase shifts in order to produce twocomparable signals. The method of. adjusting the signals by usingearphones is known in the art as the binaural listening method.

Figs. 4, 5 and 6 disclose an alternative system for Ohtaining locationsof the sources of sound. The main dilference between Fig. l and Figs. 4,5 and 6 resides in the fact that while in Fig. l the reproducing headsare normally stationary (except for the adjustment of the sectors 87 and88), and the magnetic tapes are rotated, in Figs. 4, 5 and 6, the tapesare stationary and the reproducing heads are rotated during thesignal-reproducing cycle of the system.

Referring to Fig. 4, the microphone arrays 460 and 402 are similar tothe microphone arrays 10, 12 in Fig. 1, and, as in the case of Fig. 1,they are connected to the amplifiers 485 through 410 which impress thetransformed sound waves on the recording heads 412 through 417. Therecording heads are aligned properly with respect to each other in sucha manner that the center line joining the three recording heads of theleft array is at a right angle to the longitudinal center lines of themagnetic tapes 420, 421 and 422, and the center line joining the threeheads of the right array is at a right angle to the longitudinal centerlines of the magnetic tapes 424, 425 and 426. The magnetic tapes 428,422, 424 and 426 are provided with the linear scales, as illustrated inFig. 4. The magnetic tapes are mounted on idling rollers 430, 432, andon pulleys 434 through 439, the pulleys being connected to each other bymeans of clutches 428, 429, 431 and 433. These pulleys are con nectedalso through clutches 440, 441, and gears 442, 443, and 444 to agovernor-controlled direct current motor or synchronous motor 448 whichdrives the tapes at a uniform linear velocity in the direction indicatedby the arrows. As in the case of Fig. 1, the erasing heads 458 through455 are placed in the proximity of the recording heads and thealternating current potential of supersonic frequency is impressed uponthem by an oscillator 458 for restoring magnetization of the tapes toneutral state. Ganged switches 46% and 461 are interposed between theamplifiers and the recording heads, and between the supersonic frequencyoscillator 458 and the erasing heads. From the description given thusfar, it is apparent that the recording channels disclosed in Fig. 4 areidentical with recording channels illustrated in Fig. 1.

All magnetic tapes are made to pass over idlers 464 through 475, andscanning wheels mounted between the idlers, the construction of which isillustrated more clearly in Figs. 5 and 6, Fig. 5 being the side,cross-sectional view, and Fig. 6 the vertical, transversecross-sectional view of one idler-scanning-wheel combination. Because ofthe presence of tension rollers 476 and 477, the magnetic tapes are madeto follow a considerable portion of' the cylindrical surface of theidlers as illustrated in Fig. 5. The idlers 464 through 475 arerotatably mounted on shafts 478 and 479. These shafts are connected tomotor 448 by means of belts 484 and 485 and the appropriate pulleys asillustrated in Fig. 4. The shafts are connected also to commutators 486and 487 which accomplish the same function as commutators 74 and 76 inFig. 1, except that the commutators 486 and 487 do not have theadjustable segments and 86 of Fig. 1, since the position of the selectedsignal on the screen of oscilloscopes 488 and 489 is adjusted now byshifting the tapes over the idlers and pulleys.

It should be noted here, before proceeding with the description of theadditional systems, that the system disclosed in Fig. 4 does notintroduce any appreciable error due to the slippage of the magnetictapes during the recording interval and subsequent to this interval.Experimental results indicate that such slippage is within the limits ofthe overall accuracy of the system. This is due to the fact that therecording period itself, as mentioned previously, occupies a period oftime which is in the order of at most 50 milliseconds, and since motor448 is disconnected only a few milliseconds after the expiration of therecording period, the system comes to a standstill very quickly with theresult that the slippage, if any, is insignificant.

Referring to Figs. 5 and 6 which illustrate one set of idlers 464, 465and a scanning wheel 500, the idlers are rotatively mounted on shaft 478by means of roller bearings 501 and 502, while scanning wheel 500 ispositioned between the two idlers and is keyed to shaft 478.Accordingly, the scanning wheel revolves with shaft 478. The tensionrollers 476 and 477 are rotatively mounted on arms 503.

The magnetic tape 420, during the recording process,

passes over the idlers 464 and 465 which are rotated by the tape. Thescanning wheel 50!) has a slightly smaller diameter than the idlers,with the result that the scanning wheel is free to revolve withouttouching the tape dur-' ing the recording and reproducing cycles of thesystem. The magnetic heads 504 and 505 are mounted on the rim of thescanning wheel 500 with the magnetic core of the reproducing headsprotruding through the rim of the scanning wheel, so that only a smallair gap separates the magnetic cores from the tape when they are inreproducing relationship with respect to the tape. Only two magneticheads are illustrated in Figs. 5 and 6, but the number of thereproducing heads may be increased to any desired number depending uponthe desired repetition rate of the signal. This is determined primarilyby the char acteristics of the fluorescent screens in the oscilloscopetubes 488 and 489 and the speed ratios of the driving mechanisms. Themagnetic heads are connected in series, with one terminal of the seriescircuit groundedv at 506 through shaft 478, and the other brought outthrough a slip ring 508, brush 509 and'conductor 51"0,

the latter being connected to one 'of the terminals of a three-positionswitch 490, Fig. 4. Depending upon the position of this switch, any oneof the three scanning disks may be connected through an amplifier 491 tooscilloscope 488, which reproduces the signals on its screen ina mannerdescribed in connection with Fig. 1.

The operation of the position-finding system disclosed in Figs. 4, and 6is as follows: the microphone arrays 400 and 402 are staked out, each ina substantially horizontal plane, at the ends of base line 14 and thesystem is made ready for operation by closing the circuits of allamplifiers and the ganged switches 460 and 461 which connect. themicrophone arrays to the recording heads. When the operator hears theexpected sound, he opens switches 460 and 461 which disconnect thearrays from the recording heads, and the supersonic oscillator 458 fromthe erasing heads, thus leaving the desired signal in the recorded formon all six magnetic tapes. Operation of the switches 460 and 461 alsodisconnects the driving motor 448 (the motor switch is not indicated inthe drawing), with the result that the tapes come to a standstillshortly thereafter. The displacement of the signal on the magnetic tapes420 and 422 with respect to the signal on tape 421 is measured in thefollowing manner: clutch 440 is disconnected and motor 448 is startedagain, with the result that all reproducing heads mounted on shaft 478are now revolved on their scanning disks, such as disk 500 shown in Fig.5. With the clutches 428 and 429 in the engaged positions, the threemagnetic tapes 420, 421 and 422 are moved by hand by engaging knurledknobs 492 and 495 until the desired signal appears on the screen ofoscilloscope 488, which takes place when magnetic tape 421 is moved intosuch a position that the desired signal is positioned directly overidlers 466 and 467, Fig. 6, and the reproducing heads 505 and 504 aresubjected to the influence of the transverse flux produced on tape 421during the recording process. This flux induces corresponding electromotive forces in the reproducing heads and the induced voltage isimpressed on amplifier 491 which impresses the amplified signal on thevertical deflection plates of oscilloscope 488. The image is centered onthe oscilloscope screen by using either the maximum amplitude or themaximum velocity of travel of the cathode-ray beam as a reference point.After the signal recorded on tape 421 has been properly centered on thescreen, the reading on the vernier plate 493 is noted, clutch 428 isdisconnected, and tape 420 is moved by means of knob 492 until thesignal recorded on tape 420 is reproduced on the screen of oscilloscope488 in a manner similar to that of the signal from tape 421. At thisinstant, the three-position switch 490 is on its right contact, and isconnected to conductor 510, thus connecting amplifier 491 and theoscilloscope to the reproducing heads 505 and 504. Switch 490 now may beswitched back and forth between the right and center contacts in orderto compare more carefully the exact positions of the two signals on theoscilloscope screen. When the exact over-lapping of the images isobtained, tapes 421 and 420 are properly aligned and, therefore, vernier493 may be read now once more in order to determine the lineardisplacement given to tape 420 during its adjustment to the secondposition. The same procedure is followed in connection with tape 422,clutch 429 being disconnected when tape 422 is moved to that positionwhich reproduces the recorded signal in the desired position on thescreen of the oscilloscope. Vernier 494 is used for measuring the lineardisplacements of tape 422. The readings obtained on the verniers 493 and494 are used in Equation 4 in exactly the same manner as the readingsobtained on the counters 90 and 91 of Fig. 1.

' In Fig. 4, displacement of the magnetic tapes is meas- "ured by meansof verniers 493 and 494, with the magnetic tapes having the scalesetched on their outer edges. This vernier arrangement may be replacedwith the counters arrangement used in Fig. 1, in which case,- thecounters- 91 and 92 are provided with the worms 89 and 90 normally heldout of engagement with the gears 87' and 88',

attached to pulleys 434 and 436, by coil springs. When it is necessaryto move tapes 420 and 422 for proper centering of the signals on theoscilloscope screen, the operating shafts of the counters are pressedinward against the pressure of the springs until they engage the teethof gears 87' and 88 attached to the pulleys 436 and 434, and when thegear engagement is established the shafts of the counters are turned inthe desired direction until the signals are properly centered on theoscilloscope screen. The readings of the counters are then used inEquation 4.

Fig. 7 discloses a simplified version of the positionfinding systemdisclosed in Fig. 4, the simplification resid-- ing in the reduction ofthe number of the magnetic tapes used for recording the interceptedsound wave; In Fig. 4 six magnetic tapes are used, while in Fig. 7 thesame result is accomplished by means of two tapes 708 and 702. Theremaining elements of the systems disclosed in Figs. 4 and 7 areidentical. The sound-intercepting arrays 704 and 706 are connectedthrough amplifiers to recording heads 707 through 712 which record thesignals impressed upon them by the amplifiers on different portions ofthe two tapes. The erasing heads 714 through 719, positioned in front ofthe recording heads, are connected to a supersonic oscillator 720, theerasing of the signals being performed by them directly in front of therecorded heads, which provides the necessary time reservoir on themagnetic tapes. The scanning wheels 722 through 727, with their magneticheads, are placed on the lagging sides of the recording heads. As inFig. 4, they are connected through shafts 728 and 729 to a motor 730which is also connected through shafts 732 and 733, and clutches 734 and735 to the driving pulleys 736 and 737 of the magnetic tapes. Thereproducing heads are connected to oscilloscopes 738 and 739 throughamplifiers 740 and 741 and three-position switches 742 and 743. Thesaw-tooth oscillators 744 and 745 are controlled by the commutators 746and 747 in a manner similar to that illustrated in Figs. 4 and 1. Sincein Fig. 7, during the reproducing cycle of the system, the magnetictapes may be moved by hand back and forth in order to adjust theposition of the recorded signal on the oscilloscope screen, it isunnecessary to have the adjusting segments, such as segments and 86,Fig. 1, for commutators 746 and- 747.

The operation of the system disclosed in Fig 7 does not differmaterially from the operation of the system disclosed in Fig.- 4. Themicrophone arrays impress the intercepted signal on the recording headsand the operator, upon hearing the sound, disconnects the gangedswitches 750 and 752 and a switch for motor 730 (not shown), with theresult that the desired signal is retained on the two magnetic tapes.The operator is now ready for determining the displacements of therecorded signals on the magnetic tapes. Since the magnetic tapes aremoved at. a relatively low speed, they stop quickly after motor 730 isdisconnected and the operator soon learns how much he has to move themagnetic tapes in the direction opposite to their normal rotation duringthe recording period in order to position the desired portion of themagnetic tape under the scanning wheel 724. This scanning wheel isplaced on the lagging side of the recording head 708 which is connectedto a microphone 754. It may be recalled that the measurement of theamount of displacement of the recorded signals is always performed byorienting the signal from microphone 754 first, and then measuring thedisplacements of the signals produced by the microphones 755 and 756with respect to the signal produced by microphone 754. The sameprocedure is followed in the system disclosed in Fig. 7 with the tape700 being moved by hand until the signal from microphone 754 is properlyaligned with the scanning wheel 724, by observing the screen ofoscilloscope 738 connected to the scanning wheel at this instant throughamplifier 740 and switch 742. Since all three signals are recorded onthe same magnetic tape, it may be found convenient to record the signalimpressed on the recording head by microphone 754 in 180 phaserelationship with respect to the signals impressed by the microphones756 and 755, this phase relationship facilitating the identification ofthe signals on the magnetic tape. After the signal from microphone 754has been properly aligned on the screen of oscilloscope 738, vernier 753is read, and the tape is shifted to the position which aligns the signalfrom microphone 756 with the scanning wheel 722. When this isaccomplished, vernier 758 is read once more which gives the displacementbetween the signals picked up by microphones 754 and 756. The sameprocedure is used for measuring the displacement between the signalsfrom the microphones 754 and 755, and the obtained readings are used inEquation 4.

Fig. 8 discloses an additional modification of the position-locatingsystem in which all steps are performed by electronic means, andrecording of the sound waves on the magnetic tape is eliminated. As inall prior figures, two microphone arrays S and 802, properly positionedwith respect to the base line 14, and each oriented in a horizontalplane, are used in intercepting the desired sound wave, six amplifiers803 through 308 being used for amplifying the intercepted signals. Theamplified signals from the amplifiers 803, 805, 306 and 808 areimpressed on the electronic switches 809 and 811, respectively, whichare so constructed that they key the outputs of the amplifiers connectedto them in the desired alternate sequence, and also combine the audiosignal with the rectangular waves generated by the switches. A detaileddescription of the electronic switches suitable for this purpose may befound in a pamphlet titled Electronic Switch and Square Wave Generator,type l85-A, by Allen B. DuMont Laboratories, Passaic, NewJersey. Twodouble-beam oscilloscopes are used for televising the currents flowingin three output circuits of each array. Microphone 831 is connecteddirectly to one set of the defiection plates which produce a trace 902,Fig. 9, on the screen of oscilloscope 814. For a more detaileddescription of the double-beam cathode-ray tubes reference is made toCossor Double Beam Oscilloscope, by A. C. Cossor, Ltd., London, England.The audio signals and the rectangular waves appearing in the output ofswitch 809 are impressed on the second set of the deflection plates ofoscilloscope 814, so that two separate televised" images 900 and 901 ofthe intercepted sound wave are reproduced on the screen of theoscilloscope, each image corresponding to the signal appearing in theoutput of one microphone producing it. In order to obtain such results,switch 000 keys the outputs of amplifiers 803 and 805 in alternatesuccession, so that during the firs-t portion of the rectangular wave,which is also generated by switch 809, the cathode-ray beam is tracingon the oscilloscope screen a portion of signal 900 from microphone 830.When the rectangular wave drops to a lower level, the output ofamplifier S03 is cut off, and the output of amplifier 805 is allowed topass through switch 809, with the result that during the succeedingperiod the cathoderay beam produces a portion of trace 901 correspondingto a signal from microphone 832. The switching frequency of switch 809is adjusted to produce substantially continuous traces of the twosignals 900 and 901. The oscilloscope screen must have maximumretentivity, so that the generated images may be retained on the screenfor a sufiicient length of time to allow the operator to read therelative displacements of the two traces with respect to the third trace902. The oscilloscope screens are provided for this purpose with thevertical scale lines 903. The saw-tooth oscillator represents acomponent part of oscilloscope 814 and the recurrence of the sawtoothwave is adjusted to reproduce the entire sound phenomenon intercepted bythe array in a single sweep. The circuits and functioning of the rightarray 802 are identical to the circuits of the left array.

Fig. 10 discloses an additional modification of Fig.v 1, themodification residing in the fact that the reproducing amplifier 71 ofthe left array, and 72 of the right array, Fig. 1, have been eliminatedand the recording amplifiers 22 through 27 are now used for recording aswell as for reproducing the recorded signals. Moreover, switchingarrangements interposed between the sound arrays, the amplifiers, andthe recording and reproducing heads are so arranged that the amplifiers,during the reproducing period, are interchanged or criss-crossed so thatthe previously described phase error which may be introduced bytheamplifiers is automatically cancelled during the reproducing period.Fig. 10 discloses in detail only the left array, therecording-reproducing channels of the right array being illustrated as ablock 1000, since they are identical to those of the left array, whichis described below. Since by far the larger portion of the individualcomponents of the systems disclosed in Fig. 10 is identical to thecorresponding components used in Fig. 1, they are identified by the samenumbers in Fig. l and 10, and only the added switches are identified bythe new numerals.

During the recording period, sound array 10 is connected to therecording heads 28, 29 and 30 through the amplifiers 22, 23, and 24, andclosed switches 1002 through 1008. Switch 40 is in closed position, andthe five-position switches 1010 and 1009 are both on their respectivecontacts No. 1. Accordingly, with the switches in the positionsdescribed above and indicated in the drawing, the sound array 10 isconnected to the recording heads 28, 29 and 30 through the amplifiers22, 23 and 24, and recording of the desired sound wave is made in theusual manner. Oscillator 58 is connected through switch 40 to theerasing heads 51, 52 and 53, so that the heads function in a mannerpreviously described in connection with Fig. 1. When the operator hearsthe expected sound, he opens switch 40, and 1002 through 1008, with theresult that recording of any sounds intercepted by the sound array 10and the erasing process are stopped. The desired sound wave has beenrecorded or trapped'on the magnetic tapes, and it is now necessary tocompare the phase relationship of the recorded signals by impressingthem, one at a time, on oscilloscope 68. Switch 1002 is now closed andthe multiple-contact switches 1010 and 1009 are now turned to contactNo. 3, while switches 1006, 1007 and 108 is turned to contact No. 2.Thus the outputs of amplifiers 22 are connected to oscilloscope 68 overconductor 1011, and the reproducing head 61 is connected through contact3 of switch 1009, conductor 1014, and contact 3 of switch 1010 to theoutput circuit of amplifier 22. Accordingly, the reproducing circuit ofthe magnetic head 61 is now as follows: magnetic head 61, contact 3 ofswitch 1009, conductor 1014, contact No. 3 of switch 1008, amplifier 22,contact No. 2 of switch 1006, conductor 1011 and oscilloscope 68. Itwill be noted that only one reproducing head, head 61, is connected tooscilloscope 68 at this time, the circuits of the reproducing heads 60and 62 being open at the multiplecontact switches 1010 and 1009. Itshould be also noted that while recording of the sound wave at themagnetic tape 35 has been accomplished with the aid of amplifier 23,amplifier 22 is used when the signal recorded on tape 35 is reproduced.This is the criss-crossing of the ampli fiers mentioned previously.

The same criss-crossing of the amplifiers is followed when the signalsrecorded on the magnetic tapes 34 and 36 are reproduced on the screen ofoscilloscope 68, amplifier 23 being used for reproducing the signalrecorded on tapes 34 and 36, and amplifiers 22 and 24 for reproducingthe signal recorded on tape 35. Thus the phase difference which may beintroduced by the recording amplifiers 22, 23 and 24 is now eliminatedduring the reproducing cycle by interchanging the amplifiers during thereproducpared to the signal recorded on tape 35, reproducing head 61must'be switched over to the input of amplifier 24 at this time, so thatamplifier 24 is used with head 61, while amplifier 23 is used with head62. This is accomplished by turning switches 1010 and 1009 to No. andNo. 4 positions, respectively. The multiple-contact switches 1010 and1009 are ganged switches, so that they are operated to the same contactpositions simultaneously. These switches are arranged for quickcomparison of the phase relationship between the signals recorded on thetapes, this comparison being accomplished by rapid oscillation ofswitches 1009 and 1010, first, between contacts 2 and 3, and'thenbetween contacts 4 and 5, which produces rapid flashing of either onesignal or the other on the oscilloscope screens Switches 1006, 1007 and1008 are also ganged to switches 1009 and 1010 for accomplishing thisresult.

Therintensity of the signal interceptedv by the sound array 10, in themajority of cases, is such that higher decibel gain in amplifiers 22through 24 is required during the recording cycle than during thereproducing cycle. An attenuating non-inductive resistance 1020 isinserted in series with conductor 1014 for reducing the amplitude of thesignals impressed on the amplifiers to the desired volume.

While the amplifier switching arrangement has been illustrated inconnection with apparatus similar to that disclosed in Fig.1, it shouldbe understood that it is equally applicable to the apparatus disclosedin Figs. 4 and 7.

It is believed that the construction and operation of the rangingsystems, as well as the many advantages thereof, will be apparent fromthe foregoing description. It should be understood that while theinvention has been shown in several preferred forms many furthervariations will be apparent to those skilled in the art.

What is claimed is:

1. In combination, a plurality of records, common driving meanstherefor, a translating device for each 7 record, means for adjustingthe relative angular position of said translating device with respect tosaid record, indicating means coupled to said translating device toindicate an adjustment, a reproducing circuit including a cathode-rayoscilloscope and a sawtooth generator connected to said oscilloscope, aswitch for connecting said reproducing circuit with any one of saidtranslating devices, a commutator operated by said driving means forcontrolling said sawtooth generator, means for adjusting the relativeangular positions of commutaton'translating device, and record, forselecting different portions of the reproduced record, displayed on saidoscilloscope.

2. A sound ranging system for locating the azimuth of a source of sound,including three microphones each one of which is placed at the apex ofan equilateral triangle, a recording channel connected to each of saidmicrophones,

said recording channel including an amplifier, a magnetic recording headand a magnetic tape in the form of a loop placed in recordingrelationship with respect to said recording-head, anerasinghead for eachof said recording channels, an oscillator connected to all of theerasing heads, amultiple contact switch for simultaneously disconnectingsaid microphones from said recording channels, and said oscillatorfrom'saiderasing heads; a reproducing head for each of said recordingchannels, an oscilloscope for said microphones, a switch for connectingany one of the reproducing heads to said oscilloscope, a common drivingshaft mechanically interconnecting all of said loops, and a drivingmeans connected to said shaft.

3. A system for determining the azimuth of a sound wave including amicrophone array having a plurality of microphones, a' recording channelconnected to each microphone, each of said .channels including anamplifier, a magnetic recording head, and a magnetic recording mediumfor recording said sound wave as a plurality'of magnetic records, thenumber of said records corresponding to the number of said microphones,a reproducing.

head in reproducing relationship with respect to each of said records,means connectable to said reproducing heads for measuring thedisplacements of all records except one with respect to the oneremaining record, said means including a switch for connecting thereproducing heads of the records that are being compared to the inputsof the amplifiers in the corresponding recording channels in aninterchangeable manner, such as amplifier No. 1 connected to reproducinghead No. 2, and amplifier No. 2 is connected to reproducing head No. 1.

4. A system as defined in claim 3 in which said means further includesan oscilloscope and wherein said switch also connects the output of oneamplifier at a time to said oscilloscope. 1

5. A combination including first, second and third microphones, saidmicrophones forming a microphone array, first, second and thirdrecordingchannels connected to the respective microphones, each of said channelsincluding an amplifier, a recording device, and a moving record member,first, second and third reproducing devices in reproducing relationshipwith respect to said record member, means for comparing the relatviepositions with respect to each other of the records produced by saidchannels on said record member, and a switch for connecting said firstreproducing device with the second amplifier, said second reproducingdevice with said first and third amplifiers, and said third reproducingdevice a with the second amplifier during the comparing cycle of saidcombination.

6. In a sound ranging system, a plurality of spaced microphones forpicking up sound waves from a source of sound to be located, a loop typemagnetic recording member for each microphone, recording and erasingmagnets for each record-ing member, means for driving the members torecord anderase the outputs of the microphones, means for disabling'allof the magnets to leave a record of the wave form of the sound from thesource on each of the members, visual reproducing apparatus forselectively scanning the members to identify the recorded wave forms ofsaid sound, means for bringing a plurality of the identified wave formsinto accurate visual alignment, registering devices operated inaccordance with the operation of the aligning means whereby thedifierences in arrival time and hence the location of the sound may bedetermined.

7. A system for determining the azimuth of arrival of a sound waveincluding an equilateral triangle microphone array oriented in ahorizontal plane and having a microphone positioned at each apex of thetriangle, a recording channel connected to each microphone, each of saidchannels including a magnetic recording head and a magnetic tape, saidmagnetic tapes being mounted on a common rotating shaft whereby saidsound wave is recorded on three tapes as three individual records ofsaid wave, the position of said records on said tapes corresponding totime of arrival of said wave at each microphone of said array, areproducing head for each of said tapes which may be rotated relative toits associated tape to reproduce the sound wave recorded thereon andvisual reproducing means connectable to any one of said reproducingheads, two of such tapes beingcalibrated, and means to uncouple saidtapes from said common shaft, whereby said tapes may be adjusted tosuperpose said waves and to determinethe difference in time of recordingfrom the calibrations on said tape.

8. In the method of obtaining azimuth of a sound wave by means of atriangular microphone array having three microphones positioned at threeapex points of said array respectively and a separate magnetic recordercoupled to each of said microphones to produce a magnetic record fromthe output therefrom, the steps which include: transforming said soundwave into three, separate voltage waves at said three points, the phaserelationship between measuring the phase differences between the twovoltage waves and the third voltage waverespectively by measuring theirrelative position ontwo of said records" with respect to the recordedposition on-said third recordand deriving the azimuth of said sound wavefrom the ratioof said differences, whereby said'azimuthis determinedsolely by the'relative positions 'of'said sound-waveon' said records. 1

9. The method of obtaining theazimuth of a source to each other inaccordance with the timeiof arrival of said sound wave at-said threepoints, measuring ateach station the displacementsof the two imageswith-respect to the: third image; and deriving the azimuth'line of saidsound wave from the, ratio of said displacements:

References Cited in the file of this patent UNITED STATES PATENTS1,378,960Z Horton" May 24', 1921 1,406,996 Biol [i1].,.- -;--L-; Feb.21-, 1922' 1,415,973 Allen May 16, 1922. 1,502,243. Fry July 22, 19241,645,810 Hubbard Oct. 18, 1927 2,418,136 Munson Apr. 1, 1947 2,424,773Rieber July 29, 1947 2,611,023 Dunn Sept. 16, 1952. 2,767,917 Fawcett'a........ Oct. 23, 1956

